I open it and look for some "accelCommandQueue", which is related to Metal. Clicking on a command queue will show the app name in the "Value" column of the right-hand panel.
But it 10.12, it seems that basically every apps is associated to a command queue, since the whole UI runs on Metal.
If the app uses openGL, it should be associated to some "accelGLContext". This is the case for Dear Esther and gfxbench GL, but not gfxbench Metal.
With that said, if an app uses both openGL and Metal, it may be difficult to tell how much it uses Metal in 10.12 (simply for the basic UI or 3D rendering?).